T-2 1/4 Glass Wedge Base bulbs are designated by the letter T. The T
designates that the bulb is tubular in shape. The number(s) following
the T indicates the width of the glass envelope (not the base) in
eighths of an inch. T-2 1/4 Glass Wedge Base bulbs are two and one quarter
eighths of an inch or 0.2813" or 7.1438 millimeters in diameter.